This invention relates to auxiliary wheel assemblies for over-the-road vehicles, such as trucks, which can be lowered to serve as an additional load bearing axle when required to meet highway regulations.
Auxiliary wheel assemblies are commonly used for load distribution purposes on trucks used for hauling heavy loads, both tractor-trailer combinations and trucks. When dead-heading, hauling a light load or a tractor is driven alone, such auxiliary wheel assemblies typically are arranged so that the auxiliary wheels can be raised out of contact with the ground to eliminate unnecessary tire wear.
In many instances, the applicable government regulations require the axle for the auxiliary wheels to be spaced some distance from the rear or drive wheels. Such spacing can cause a certain amount of lateral slippage or tire scuffing when curves and corners are being negotiated. This problem can be minimized by making the auxiliary wheels steerable. If the auxiliary wheels happen to be turned when they are lowered, tire scuffing and other damage to the assembly can occur during initial movement of the truck before the wheels are straightened. This is particularly true in the event the auxiliary wheels are in a severely turned position.
Pressurized air bags or springs are particularly desirable for use in suspension systems for auxiliary wheel assemblies because of their capability of providing a substantially constant force on the auxiliary axle. Such air springs typically are located between the longitudinal members of the truck frame and the auxiliary axle which quite often is the most convenient location from the standpoint of simplifying the mounting structure and the amount of space required. However, when the air springs are so located in prior constructions, they tend to limit the extent the auxiliary wheels can be raised above the ground to only a few inches. Consequently, when the truck is being driven over the uneven ground with the auxiliary wheels raised, they can contact high areas and lift the vehicle, in which case the steering or drive wheels can become ineffective because they are not in firm contact with the ground.
Exemplary prior constructions of auxiliary wheel assemblies including steerable wheels and/or air bag type suspension systems are disclosed in U.S. Pat. Nos. 3,390,895, 3,447,738, 3,704,896, 3,771,812, 3,895,818 and 4,084,833 and Canadian Pat. No. 961,516. None of these patents is concerned with the problem of the auxiliary wheels being turned when lowered into contact with the ground.
With respect to increasing clearance, U.S. Pat. No. 3,704,896 proposes locating the air bags beside the longitudinal frame members and making them longer in order to increase the distance the auxiliary wheels can be raised above the ground. U.S. Pat. No. 4,082,305 proposes arranging the auxiliary wheel assembly so that the axle thereof rides behind the rear end of the vehicle frame and can be lifted above the lower edges of the vehicle frame to provide additional clearance.
U.S. Pat. Nos. 3,420,542, 3,920,262 and 4,213,626 respectively disclose a damper for the front steering wheels of a vehicle, a spring arrangement for counteracting the tendency for vehicle steerable wheels to bow outwardly under weight, and a spring arrangement for maintaining vehicle steerable wheels in a neutral roadway traveling position to compensate for misalignment due to wear to the steering components.
Another common shortcoming of prior steerable auxiliary wheel assemblies is a tendency of the auxiliary wheels to turn in the wrong direction or become "jackknifed" during backing. That is, for auxiliary wheel assemblies located in front of the vehicle drive wheels, the auxiliary wheels tend to become turned to the left when the vehicle is being backed with the front wheels turned to the right and vice-versa. For auxiliary wheel assemblies located behind the vehicle drive wheels, the auxiliary wheels tend to become turned in the same direction as the front wheels during backing instead of the following the rear wheels. Consequently, it is usually necessary to raise the auxiliary wheel assembly before backing.